Influenza, a common respiratory pathogen, causes significant morbidity. Healthy children <2 years of age have comparable influenza-attributable hospitalization rates to persons >65 years of age. With greater appreciation for this pediatric morbidity, influenza vaccine recommendation for children has been expanded. The first specific recommendation for healthy children was the 2002-04 recommendation to "encourage influenza vaccine for all children 6-23 months of age when feasible." Beginning in 2004-05, influenza vaccine was fully "recommended for all children 6-23 months of age." The impact of these new recommendations on influenza vaccination rates of children needs to be evaluated. We hypothesize that higher influenza vaccination rates will lead to fewer influenza hospitalizations and outpatient visits. The overall goal of this proposal is to evaluate the impact of pediatric influenza vaccination rates on the epidemiology of influenza hospitalizations and outpatient visits. To examine these hypotheses, we plan to utilize the infrastructure of the New Vaccine Surveillance Network (NVSN) created by the Centers for Disease Control in 1999. Three NVSN sites collect population-based, laboratory-confirmed, prospective surveillance data on acute respiratory viral infections in young children and are funded for a second 5-year cycle. As stated in the letter from our CDC Project Officer, NVSN funds the surveillance infrastructure and is very supportive of investigators seeking external funding to evaluate the NVSN database, such as the comprehensive influenza studies proposed by Dr. Poehling. This proposal addresses three specific aims: 1) To prospectively determine the population-based rate of laboratory-confirmed influenza hospitalizations among children <5 years of age before (2000-04) and after (2004-08) the new influenza vaccine recommendations in three regions of the U.S., 2) To determine the impact of laboratory-confirmed influenza infections on outpatient visits in outpatient clinics and emergency departments among children <5 years of age before (2002-04) and after (2004-08), and 3) To prospectively assess influenza vaccination rates among children <2 years of age. The proposed research and career development plan complement Dr. Poehling's training in Pediatrics and Epidemiology. Furthermore, the proposal supports her efforts to obtain expertise needed to become an independent investigator focused on reducing pediatric morbidity from influenza infections.